Method of effecting x-ray analyses

ABSTRACT

To the anode of an X-ray tube a basic direct current voltage is applied, approaching the threshold voltage required for the excitation of the analyzed element, and superimposed on this voltage is a sinusoidal alternating voltage. In the positive portions of the sinusoidal voltage the combined voltage exceeds the threshold voltage, whereas in the negative portions it lies below the same. The contents of the analyzed element is registered by a detector whose electronic evaluating device is selectively sensitive to the third or higher harmonics.

United States Patent Voparil [54] METHOD OF EFFECTIN G X-RAY ANALYSES Rastislav Vopari], Czechoslovakia Assignee: Chirana Modmny narodiri podnik,

Prague, Czechoslovakia Filed: Jan. 22, 1970 Appl. No.: 4,978

Inventor:

[30] Foreign Application Priority Data Jan. 27, 1969 Czechoslovakia ..5] l-69 U.S. Cl ..250/5l.5, 250/49.5 PE, 250/102 Int. Cl. ..G0ln 23/22 Field of Search ..250/49.5 A, 49.5 PE, 51.5,

[56] References Cited UNITED STATES PATENTS 2,745,019 5/! 956 Hamacher ..2S0/l02 X MODUL A 70 I 1 3,701 ,899 1 Oct. 3 1, 1972 Alvarez ..250/5 1 .5 Ziegler ..250/49.5

Primary ExaminerWilliam F. Lindquist Attorney-Arthur O. Klein [57] ABSTRACT 2Claims,7DrawingFigures .2: rec rm? ME an: a? A 4 c 4 MP4 0 /5,? (55: 6 7/ V5 TO HA EMO/V/CS) PMENTED am a 1 m2 SHEETSNI INVENTOR: agasfisla v V642. 01;

ATTORNEY PATENT ED 31 SHEET t 0F 4 4b 50 in sec INVENTOR Rasb'slav VOP/IR/L BY ATTORNEY METHOD OF EFFECTING X-RAY ANALYSES BACKGROUND OF THE INVENTION 1 Field of the invention The present invention relates to a method of and an apparatus for efi'ecting X-ray analyses, namely spectral as well as diflraction analyses. Spectral analysis is employed for determining a chemical element in a sample, by means of a characteristic X-ray radiation emitted by the sample. it the sample is excitated by X-ray radiation from an X-ray tube, a secondary analysis is concerned. If the sample is excitated directly by the incidence of electrons, a primary analysis is spoken of. in the event of the X-ray radiation from the X-ray tube passing through the sample, an absorption analysis is effected.

A further type of analysis is termed "diffraction analysis," by means of which the fine structure of a sample is determined, regardless of the element contained in the sample.

2. Description of the Prior Art The analyzers known up to now employ a so-called analyzing monocrystal for determining the wavelengths of radiation emanating from a sample; the analyzing monocrystal is mounted for rotation, so that at various incidence angles of the analyzed radiation it fulfills successively the conditions for reflexion given by Braggs equation for difi'erent wavelengths. A detector receiving the reflection from the crystal under various angles registers in this way successively the diffracted radiation of various wavelengths. This radiation, however, has a low intensity and therefore requires sensitive detectors, such as Geiger-Miiller counters or a detector of the proportional, scintillation or semiconductor type, registering in the form of individual impulses every quantum of X radiation absorbed in the active zone of the detector. An electronic device, connected to the detector, counts the impulses, either all of them or, selectively, only those impulses which have an amplitude of a required magnitude and, finally, it evaluates them as a number of impulses during a predetermined measurement period or, in addition, effects their registration.

These known methods and appliances are characterized by the fact, that electrons serving for the excitation of the sample or for generating the exciting X-ray radiation in an X-ray tube, are accelerated by a voltage substantially higher than that corresponding to the threshold excitation voltage of the analyzed element. This is necessary for achieving a sufiicient intensity of the X-ray radiation. The shape of the accelerating voltage is in principle of no consequence, as it can be a rectified, pulsating or smooth voltage.

These analyzers, employing analyzing crystals, sufier from many disadvantages, in particular the low intensity of the registered X-ray radiation caused by the low reflecting effect of the analyzing crystal and the unavoidably large distances between the sample and crystal and then between the crystal and detector. This low intensity is the cause of a considerable random fluctuation of the measured values and calls for longer measuring times. Moreover, several kinds of exchangeable crystals with different lattice parameters are required, if elements in a wide range of atomic numbers are to be analyzed and an undesirable overlapping of the lines K: and K B of the two elements with neighboring atomic numbers, as well as duplicities caused by reflections of higher orders are encountered. The efforts to dispense with the analyzing crystals by employing amplitude modulation of the registered impulses by electronic means, have not met with success due to the insufficient ability of the hitherto known detectors to discern between elements having near atomic numbers.

SUMMARY OF THE INVENTION The present invention aims at removing the disadvantages of the heretofore known methods and devices and at efi'ecting an X-ray analysis without the use of analyzing crystals, while achieving a sufficient resolving power.

The method of effecting an X-ray analysis according to the present invention consists therein that to an X- ray tube a basic direct voltage is applied approaching the threshold voltage required for the excitation of the analyzed element a spectral analysis for excitation of the element of the anode in a diffraction analysis, and superimposed on this voltage is a sinusoidal alternating voltage which, together with said basic direct voltage in the positive portions exceeds said threshold voltage, whereas in the negative portions it lies below said threshold voltage.

According to a further feature of the invention the X-ray radiation is transformed into electric current, the dc. component, the alternating current with the frequency of the superimposed sinusoidal voltage and the second harmonic of said current are suppressed, at least one of the higher harmonics, beginning with the third, is amplified and the resulting current indicated.

The invention relates further to an X-ray analyzer for carrying out said method, the analyzer comprising an X-ray tube, a detector and an electronic evaluating device, if necessary with a counter, said X-ray tube being attached to a source of high dc voltage. The main feature of the new apparatus resides therein that in the circuit of the source of high dc. voltage, having a value approaching the threshold voltage required for the excitation of the analyzed element, or of the element of the anode, there is a modulator serving for sinusoidal modulation of said high dc. voltage of the source.

The electronic evaluating device of the detector is selectively sensitive to a frequency exceeding double the basic frequency of said modulating sinusoidal voltage.

As follows from the above, the invention is concerned with feeding the anode of an X-ray tube, which represents the source of a continuous radiation, serving for the analysis of the sample, with a smoothed high voltage, approaching the threshold voltage of the K- or L- edge of the excitated sample element to be analyzed. This smoothed high voltage is modulated by (or superimposed thereon is), a small-scale sinusoidal alternating voltage, which in the positive half-waves excites the element, whereas this does not occur during the negative half-waves.

The radiation emanating from the sample (or anode) and received by the detector contains as its time func tion primarily a dc. component, further an alternating component whose basic frequency originates from the continuous stray radiation of the sample and from the characteristic radiations of elements having a lower threshold voltage than the element undergoing analysis, further an alternating component whose basic frequency originates from said analyzed element and finally higher harmonics. In accordance with Fouriers analysis, such harmonic frequencies, if higher than the second harmonic, are present to a higher degree only in the presence of an element whose threshold excitation voltage equals the anode voltage of the X-ray tube.

The electronic evaluating device cooperating with and following the detector is selectively sensitive to the alternating current component and more particularly to one or more higher harmonics, beginning with the third harmonic; the detector indicates on a counter a deflection, depending on the contents of the analyzed element in the sample.

The invention affords a number of advantages. In the first place there is a considerable intensity of the detected radiation, simple wiring as well is used as simple components for building up the apparatus, and, in particular, expensive analyzing crystals are fully dispensed with. Moreover the overlapping of Ka and K [3 lines of two elements having neighboring atomic numbers and duplicities caused by reflections of higher order are avoided.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be clearly understood and readily carried into effect, reference will now be had to the accompanying drawings, in which FIG. 1 shows in a block diagram the arrangement of an apparatus for effecting a spectral secondary X-ray analysis.

FIG. 2 shows in a diagrammatic representation the general idea underlying the invention.

FIG. 3 is a block diagram of an apparatus for effecting an absorption analysis,

FIG. 4 is a similar view showing the arrangement of an apparatus for spectral primary analysis,

FIG. 5 represents diagrammatically an apparatus for the monochromatization of radiation in a structural analysis,

FIG. 6 shows in detail the overall arrangement of the wiring in an apparatus shown diagrammatically in FIG. 1

FIG. 7 represents a practical example of analyzing an iron-nickel alioy, explaining the operation of the apparatus as well as the generation of the various harmonics.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 2, a dc. component Va of a high voltage KV (in the following to be called basic d.c. voltage") is adjusted on the anode of an X-ray tube in such a way as to produce a voltage approaching or equalling the threshold voltage V0. The threshold voltage of an element is the minimum voltage at which the element begins to be excited and to emit its own characteristic radiation. This voltage determines what is termed the K- or L-edge of the analyzed element. According to the invention a small scale alternating voltage of a sinusoidal shape is superimposed on said basic dc. voltage Va, said alternating voltage having an amplitude v0 and basic frequency fi. The basic d.c. voltage Va should, as far as possible, equal said threshold voltage, but may deviate therefrom by not more than sufficient to comply with the relation absolute value of (Va Vo) v0 As a result, during the positive half-waves of the alternating voltage the total voltage i.e. d.c. voltage alternating voltage) exceeds the threshold voltage Vo, while during the negative half-waves it lies below the latter. During the positive half-waves the sample is therefore excited i.e. emits its characteristic radiation, while during the negative half-waves no radiation is emitted. The entire radiation is received by a detector placed nearby, with the result that a high intensity of radiation is achieved because there is no monocrystal interposed. It follows further from FIG. 2 that the size of the amplitude of the alternating voltage permits one to chose and adjust on the one hand discerning ability between two elements of near atomic numbers and, on the other hand, the intensity of radiation and sensitivity of the method, as will be clearly understood from the ensuing disclosure.

The practical application of the new method will be described later after explanation of the arrangement of the apparatus, by means of which the new method can be carried into effect.

FIG. 1 shows diagrammatically one example of an apparatus for effecting a spectral secondary fluorescent X-ray analysis. A sample 1 receiving radiation from an X-ray tube 5 is thereby excited and emits a characteristic radiation, which is fed to a detector 2. The anode of the X-ray tube 5 is supplied with a high dc. voltage from a source 3. Superimposed on this high dc. voltage by a modulator 4 is an alternating voltage, as explained above and shown in FIG. 2. The radiation from the sample I, received by the detector 2, is evaluated in an electronic evaluation device 6, which is sensitive to one or more higher harmonics, beginning with the third. Attached to the output of the evaluating device 6 is a suitable indicating means 7 such as an indicator or a recorder.

FIG. 6 illustrates in detail the arrangement of electric circuits in the aforementioned apparatus, as an example of using the invention for a secondary spectral X- ray analysis.

The high voltage source 3 feeding the anode of the X-ray tube 5 comprises an adjustable-ratio autotransformer 10 for adjusting the required voltage on the primary winding of a high voltage transformer 11, whose alternating voltage is rectified behind the secondary winding by a high voltage rectifier l2 and smoothed by a capacitor 13. The actual value of the voltage across the X-ray is measured by a volt-meter 14 with a seriesconnected resistor 15. The anode current of the X-ray tube is measured by a milliameter l6 and controlled by an adjustable resistor 17 inserted in the primary circuit of the filament transformer 18.

The superposition of the alternating voltage on the basic dc. voltage is effected by the modulator 4 which, in its simplest arrangement, comprises a transformer 19 fed from the ac. mains 20 over an adjustable-ratio autotransformer 21.

The continuous X-ray radiation, produced in the focus on the anode 22 of the X-ray tube 5, is limited by a diaphragm 23 and impinges on the sample 1, from which all radiation components are received by the detector 2.

The detector may be of the gas-type, such as a Geiger-Miiller counter or a proportional counter or of the scintillation or semiconductor type. In the example shown in FIG. 6 a conventional scintillation detector is used, comprising a fluorescent substance or foil 24, screened off against outer light by a foil made of aluminum or beryllium, further a photo-multiplier 26, whose photo-cathode 27 is fed with high voltage, e.g. IOOOV, supplied by a conductor 28. The detector dynode 29 comprise dividing resistors 30 for the various stages and a resistor 31 for the plate 32 is provided in the detector. The signal, emitted by the detector, is passed by a screened conductor 33 into an electronic evaluation device 6.

In the example shown the electronic evaluation device comprises a two-stage amplifier of alternating voltages, with a selective sensitivity for a frequency tuned by two LC-resonance circuits 34 and 35 in the grid and anode circuit of the first amplifying electron tube 36. An output transformer 37 allows the passage of the alternating component of the anode current of the second electron tube 38 only. The resulting alternating current is rectified by a diode 39 and fed to the d.c. indicating means 7, such as a milliameter or a recorder. The fluctuation of measured values can be reduced by increasing the time constant by means of a capacitor 41. The other half-wave of the alternating output current is fed to a resistor 42 over a second diode 43.

The other parts of the wiring are conventional A coupling condenser 44 permits the passage of alternating components only. A conductor 45 being attached to a plus-voltage, the anodes of both electron tubes and the second grid are supplied with current and the second grid is fed over a resistor 46. The required negative bias of the electron tube grids is obtained by the voltage drop on resistors 47, 48 and blocking capacitors 49 and 50.

Although FIG. 6 and the above disclosure refer to a particular embodiment of the new apparatus, it will be clear to those skilled in the art, that numerous modifications may be effected in connection with various circuits or the arrangement of their parts without departing from the scope of the present invention.

In order that the invention and the mode of operation be still better understood, reference will now be made to FIG. 7 of the accompanying drawings, showing a set of diagrams relating to the determination of the contents of iron in nickel by means of a secondary X- ray spectral analysis.

The threshold voltages for the emission of K-series radiation of the two elements, namely Fe (7,1 KV) and Ni (8,3 KV) are represented in FIG. 7 by straight lines. The d.c. anode voltage across the X-ray tube is adjusted so as to equal the threshold voltage of the element to be determined, i.e. VoFe 7,1KV. Superimposed thereon is a modulating sinusoidal voltage of normal mains frequency and an amplitude l KV, so that the time function of the anode voltage of the X-ray tube is given by a curve marked A in FIG. 7, which is a sinus curve.

The curve B in FIG. 7 represents the intensity of radiation, emitted by the tungsten anode of the X-ray tube, when neglecting the absorption of radiation by the window and assuming a constant current flow through the X-ray tube. According to the known law governing the generation of continuous radiation, the latter is proportional to a square of the anode voltage Le. the momentary values of the curve 8 equal the square of the values of the curve A. The curve B is not an exact sine curve any more, but contains also the second harmonics, as is apparent from the curves C produced by resolution of the curve B into two components, namely an alternating component having the basic frequency of the modulator and the second harmonic. Higher harmonics than the second harmonic in the intensity of radiation emanating from the X-ray tube, are not present, which fact can be proved easily, even by calculation.

The nickel, present in the sample, cannot be excited because the curve A of the anode voltage does not reach, at any moment, the threshold value for nickel VoNi 8,3KV.

If no iron is contained in the sample, the detector 2 receives only the radiation scattered by the sample, whose intensity is proportional to the primary radiation impinging on the sample, which means that the curve B C applies also for the radiation entering the detector, the only difference being in the degree of intensity. In this case the registered intensity contains the d.c. component, further the basic frequency f, of the modulator and the second harmonic 2 f,. The indicator 7 indicates an intensity equal zero, because the amplifier is not sensitive to frequencies from 0 to 2f,

If, however, iron is contained in the sample, there is produced in addition to the scattered radiation, given by the aforementioned curve B or C, a characteristic radiation of the element Fe, produced by secondary excitation. However, this radiation is generated in the positive halfwaves of the anode voltage only, when the threshold voltage VoFe=7, l KV is exceeded. Its intensity is approximately proportional to the square of the value by which the threshold voltage is exceeded and is given by the curve D. The radiation is interrupted in each second half-period, so that the curve D contains, in addition to the d.c. component, basic frequency and second harmonic, still a considerable amount of higher harmonics (3rd, 5th, 7th and further). These harmonies are taken up by the detector and registered by the indicator 7, as the amplifier of the evaluation device 6 is selectively sensitive to one or more of these higher harmonics.

The invention can be used not only for a spectral secondary fluorescent X-ray analysis, but also for a primary analysis, as well as for absorption analysis.

FIG. 3 shows the arrangement of the apparatus for carrying out an absorption analysis. The various parts of the apparatus are similar to those described in connection with FIGS. 1 and 6 and are therefore marked with the same reference numerals.

In this case the primary radiation emitted by the X- ray tube 5 proceeds directly through the sample 1 into the detector 2.

FIG. 4 shows the arrangement of the apparatus for efiecting a primary analysis. This sample 1 forms here part of the anode of the X-ray tube 5 and is excited directly by the impact of the electrons emitted by the cathode of the X-ray tube. The radiation emitted by the anode or sample 1 is fed directly into the detector 2.

FIG. 5 shows the arrangement of the apparatus for effecting monochromatization of the radiation, if a structural analysis is carried out. The anode of the X- ray tube 5 contains an element emitting the required characteristic radiation, necessary for effecting the diffraction structural analysis. The dc. component of the high voltage Va is again equal to the exciting edge of the element contained in the anode. The produced modulated radiation is used as a primary beam for a recording structural diffractometer, marked 8 in FIG. 5. The diffractometer 8 comprises the sample I mounted for rotation and the detector 2 placed on a rotatable protractor arm 9. The speed of angular displacement if the detector 2 is double the speed of angular displacement of the sample 1. When effecting the measurement, the detector 2, with the protractor arm 9, is continuously rotated, along with the sample 1, and in the sought reflection angles the so-called diffraction lines are registered. According to the invention also in this case the electronic evaluation device 6 is selectively sensitive to the third and higher harmonics.

The disclosed arrangement is advantageous in that the monochromatization occurring in this case as a reduction of the continuous background, is achieved without any intricate adjustment of the geometrical parameters of the system X-ray tube focus monocrystal sample.

It follows from the above disclosure that by the present invention any desired high discerning power may be achieved. This discerning power is given by the amplitude of the superimposed alternating voltage according to the relation From FIGS. 2 and 7 it is evident, that two elements may be discerned, if the difference of their threshold voltages marked A V is equal to or greater than double the amplitude v0.

I claim:

1. A method of effecting X-ray analysis of a sample containing a specified element, comprising the steps of:

a. applying across the anode of an X-ray tube a basic direct current voltage approaching the threshold voltage for the production of X-rays having an energy corresponding to the excitation edge of the element analyzed in the sample,

b. superimposing on the said basic direct current voltage a sinusoidal alternating current voltage which, together with the basic direct current voltage, in the positive portions exceeds said threshold voltage, while in the negative portions it lies below said threshold voltage,

0. exciting a sample by a beam of X-radiation emitted by the anode of said X-ray tube so that the sample emits fluorescent X-radiation,

converting within a detector the excited fluorescent X-radiation of the sample into electric current,

e. suppressing the direct current component of said current plus the alternating current with the frequency of the superimposed sinusoidal voltage plus the second harmonic of said current,

f. amplifying at least one of the higher harmonics of the resulting current, beginning with the third, and

g. measuring the amplified current as a measure of the quantity of the specified element contained in the sample.

2. A method of effecting X-ray analysis of a sample 2. app 53%31322832' 3? lil lii iilifi 52 direct current voltage approaching the threshold voltage for the production of X-rays having an energy corresponding to the absorption edge of the element analyzed in the sample,

b. superimposing on the said basic direct current voltage a sinusoidal alternating current voltage which, together with the basic direct current voltage, in the positive portions exceeds said threshold voltage, while in the negative portions it lies below said threshold voltage,

c. passing a beam of X-radiation emitted by the anode of said X-ray tube through the sample into a detector of X-radiation,

d. converting the X-radiation in the detector into electric current,

e. suppressing the direct current component of said current plus the alternating current with the frequency of the superimposed sinusoidal voltage plus the second harmonic of said current,

f. amplifying at least one of the higher harmonics of the resulting current, beginning with the third, and

g. measuring the amplified current as a measure of the quantity of the specified element contained in the sample.

* tk ti 

1. A method of effecting X-ray analysis of a sample containing a specified element, comprising the steps of: a. applying across the anode of an X-ray tube a basic direct current voltage approaching the threshold voltage for the production of X-rays having an energy corresponding to the excitation edge of the element analyzed in the sample, b. superimposing on the said basic direct current voltage a sinusoidal alternating current voltage which, together with the basic direct current voltage, in the positive portions exceeds said threshold voltage, while in the negative portions it lies below said threshold voltage, c. exciting a sample by a beam of X-radiation emitted by the anode of said X-ray tube so that the sample emits fluorescent X-radiation, d. converting within a detector the excited fluorescent Xradiation of the sample into electric current, e. suppressing the direct current component of said current plus the alternating current with the frequency of the superimposed sinusoidal voltage plus the second harmonic of said current, f. amplifying at least one of the higher harmonics of the resulting current, beginning with the third, and g. measuring the amplified current as a measure of the quantity of the specified element contained in the sample.
 2. A method of effecting X-ray analysis of a sample containing a specified element, comprising the steps of: a. applying across the anode of an X-ray tube a basic direct current voltage approaching the threshold voltage for the production of X-rays having an energy corresponding to the absorption edge of the element analyzed in the sample, b. superimposing on the said basic direct current voltage a sinusoidal alternating current voltage which, together with the basic direct current voltage, in the positive portions exceeds said threshold voltage, while in the negative portions it lies below said threshold voltage, c. passing a beam of X-radiation emitted by the anode of said X-ray tube through the sample into a detector of X-radiation, d. converting the X-radiation in the detector into electric current, e. suppressing the direct current component of said current plus the alternating current with the frequency of the superimposed sinusoidal voltage plus the second harmonic of said current, f. amplifying at least one of the higher harmonics of the resulting current, beginning with the third, and g. measuring the amplified current as a measure of the quantity of the specified element contained in the sample. 